The present invention pertains to a new procedure for producing filtration agents by calcination of diatomites in a circulated bed furnace, by means of which it is possible to control the agglomeration of the diatomaceous particles and the cristobalite content of the finished product.
Diatomites are sedimentary rocks that result from the accumulation over geological epochs, notably the tertiary era and the quaternary era, of the fossilized remains of diatoms. Diatoms are unicellular algae that developed, and still develop in modern times, in lakes, rivers and oceans. Their protective envelopes have a siliceous framework of very variable form, resembling that of rosettes, combs, mats, muffs, etc., but always with alveoli and extremely fine channels. Diatomites are thus siliceous rocks of very low density and with a specific surface area ranging from 1 to 40 m.sup.2 /g. Because of these very specific characteristics, numerous industrial applications have been developed for diatomites for many years, notably as filtration agents, fine fillers for paints, catalytic supports, etc.
Diatomites constitute a mineral which is extracted from their natural deposits and then must be transformed into industrial products. The ore undergoes various preliminary treatments: crushing, drying, grinding and elimination of large impurities (basalt, clays, sand, etc.). It is then subjected to the important calcination treatment, the purpose of which is to diminish the moisture content from 60% to circa 1%, burn the organic materials that it contains, and to transform into oxides, silicates or aluminosilicates certain undesirable mineral compounds that ordinarily accompany the silica such as calcium carbonate, calcium sulfate, iron derivatives and possibly sulfides.
When filtration agents are being produced, special attention must be directed to the permeability which is obviously an essential characteristic of this type of product. This permeability is usually measured in darcys, with the darcy being the permeability of a porous bed 1 cm high and with a 1 cm.sup.2 section through which flows a fluid with a viscosity of 1 mPa.s (or 1 centipoise) with a flow rate of 1 cm.sup.3 /second under an applied pressure differential of 1 atmosphere; one darcy is 0.987 10.sup.-12 m.sup.2, i.e., more or less 1 pm.sup.2. Industrial filtration agents, also referred to as filter aids, generally have permeabilities between 20 millidarcys and 15 darcys, notably higher than those of the diatomites which are the raw materials. Calcination of diatomites for the production of filtration agents therefore also has the goal of agglomerating the diatoms and their fragments of several micrometers in length into aggregates of 10 .mu.m or more, so as to reduce the content of fines and to increase the permeability. Obviously, this is a partial agglomeration which must be managed so as to avoid complete fusion of the diatomaceous skeletons and formation of aggregates of a size greater than 50 .mu.m which have the serious drawback of decanting or depositing in the low points of the pipes or filters during filtration operations. It is also appropriate during this treatment phase to prevent occurrence of crystallization of the amorphous silica which is the essential contituent of diatomite. Thus, the technical problem to be resolved is the production of filtration agents by means of a diatomite calcination operation that destroys its natural impurities, develops a correct permeability in the calcined diatoms while maintaining their degree of crystallinity (cristobalite, quartz or tridymite) at a very low level.
It has actually been determined (Kadey, 1975; Deer, 1966) that when diatomaceous earths contained, depending on the deposits, less than 1% cristobalite, their calcination induced the appearance of cristobalite and to a lesser degree of tridymite by thermal conversion of the amorphous silica that is the essential constituent of the diatoms' skeleton. The content of cristobalite in the products obtained by simple calcination ranges from 1 to 100%, depending on the raw material and the temperature; it reaches 40 to 80% in so-called white filtration agents, derived from calcination in the presence of fluxes such as sodium carbonate. It is known that the inhalation of crystalline silica dust can induce silicosis, a serious pulmonary disease. A monograph by the Internal Agency for Research on Cancer (IARC) "Evaluation of carcinogenic risks of chemicals to humans, Silica and some silicates", Volume 42, 1987 even implicated crystalline silica as a potential carcinogen. This opinion is highly controversial, but, no matter what the outcome might be and even though the very complex regulations governing the work considtions of personnel exposed to the risks of inhaling crystalline silica dust are strictly applied, the limiting of the crystalline silica content of diatomaceous filtration agents supplied to the agricultural-alimentary industry remains a noteworthy concern of the manufacturers. The fine cristobalite content is between 1 and 100% in the products obtained in accordance with the prior art.
In fact, in these conventional procedures for manufacturing filtration agents, a calciner temperature between 900.degree. and 1200.degree. C. is indispensable for inducing the agglomeration required to obtain a range of filtration agents with permeabilities between 20 millidarcys and 15 darcys. Now, with raw materials rich in iron, sodium and calcium, the surface fusions obviously promoted the desired agglomeration but they also trigger rapid crystallization of the silica into cristobalite. This crystallization remains at a low level below 850.degree. C. but increases very quickly above 900.degree. C. It is estimated that the critical level of impurities catalyzing this crystalline transformation is on the order of 1%.
In the old conventional procedures in which the high-temperature residency times are necessary, greater than several seconds or even several minutes, the massive formation of cristobalite is not avoided. It is true, however, that the primary concern was the physical properties of the finished products. Previously, the heat treatment of diatoms was performed by putting them on horizontal trays on which they were calcined. The product was moved from the top trays towards the bottom trays by rabble arms made of refractory cast iron. The low productivity of this system and its high maintenance cost led industrialists to prefer rotary calcination furnaces. These rotary furnances are generally very long (30 meters or more) and operate at temperatures between 1000.degree. and 1200.degree. C. (see for example Diatomaceous Earth, William Q. Hull, in Industrial and Engineering Chemistry, February 1953, pp. 258-269). The operation must be controlled with the greatest possible precision in order to avoid densification of the silica and destruction of its arachnean structure with which its industrial properties are linked.
The manufacturers have good control of this rotary furnace calcination technology. Nevertheless, the technique involves mass calcination in the presence of a flame with extremely irregular heat transmission, resulting in the fact that not all of the diatoms receive the same heat treatment. If it is desired that the heat treatment operate in the entire mass at a minimum set temperature and for a period of time that is sufficient to achieve the desired agglomeration, it is necessary to accept the fact that a portion of this mass has been subjected to surface overcalcination. This surface fusion is desirable for achieving agglomeration but must be managed so as to limit to the extent possible the destruction of the fine structure of the diatoms and the crystallization of the silica. If not, these damaged diatoms that have become excessively agglomerated hard silica particles must be ground, which unfactorably increases the "cake density" of the final product. This "cake density" is an important characteristic of filtration agents; it is the dry apparent density of the filtration agent bed that remains on a Buchner filter after filtration of a liquid in which the said filtration agent was previously suspended. It is estimated that a "cake density" greater than 0.45 g/cm.sup.3 corresponds to products that are too fused or too ground and that have lost the characteristic porous structure required for good quality filtration agents. Grinding increases the content of fines, the excessive lightness of which will be a source of filter clogging and a cause of undesirable clouding of the filtered liquids. This content of fines must be diminished by difficult, costly postreatments.
For a given raw material, the final characteristics of the calcined product, notably its permeability and
50-.mu.m residue rate, are strongly influenced by the selection of the calcination temperature and by the residency time of the diatoms in the furnace. The presence of a 50-.mu.m residue, which is standard for the products produced by the calcination procedures in accordance with the prior art, requires the manufacturer to break and grind these products when they come out of the furnace. This is a very heavy industrial constraint. In addition, grinding presents the drawback of lacking selectivity. Although it reduces the dimensions of the large agglomerates, it also reduces the dimensions of the independent diatoms, thereby resulting in an undesirable increase in the content of fines and in the "cake density" of the finished products. With the traditional rotary furnace calcination procedures or the old calcination procedures, notably upright furnaces with superimposed trays or so-called fluidized bed furnaces, which are calcination techniques in which the minimum residency time is on the order of several minutes, it is not possible to control this agglomeration and these procedures always turn out calcined products with a 50-.mu.m screen residue greater than 5% as soon as the calcination temperature exceeds 950.degree. C.
More improved procedures, such as those described in French Patent No. 2,586,588 or East German Patent DD 266,034, have employed turbulent bed furnaces, the geometry of which is such that the hot gases carry the diatomite in spiral streams. In this system, there is better control of the overall residency time of the diatomite in the gaseous stream, but it is not possible to manage either the thermal history of the diatomaceous particles or the resultant agglomeration, such that in practice the granulometry of the diatomite to be treated is determinant, which is not necessarily the granulometry that it is desired to obtain.